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    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
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    <!-- hhhh -->
    <script>
        // 变量,需要提前声明 var let const
        var a = 10;
        //var不推荐用
        let b = 11;
        const c= 12;
        //1.不能用关键字命名

        //2.命名有规范
        //常见写法,还是以字母开头,混合数字,
        //3.大驼峰StudentID小驼峰studentID,Student_id
        let _jsjs = 10;

        //常量,不可以提前声明,只能初始化一次
        const con = 10;

        let num = 10;
        num = 10;

        console.log(con)


        //变量升格
        var number;
        {var num2 = 199;
            let num3 = 199;
        }
        console.log(number)
        
        // console.log(num3)
        number = 10;
    </script>
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